1. Field of the Invention
The present invention relates to the communications arts. More particularly, this invention pertains to a system and circuitry for tracking the phase of a variable-frequency carrier signal.
2. Description of the Prior Art
In communication, the detection and decoding of transmitted information commonly include, as a necessary receiver function, the tracking of a carrier signal. Often the frequency of the carrier undergoes some variation with time, complicating the information detection process either inadvertently (e.g. though environmental variations such as temperature, etc.) or intentionally (as the result of doppler shift.)
Present-day receiver systems invariably utilize digital computer-aided information processing. Circuits must be utilized in conjunction with such data processing apparatus for tracking the carrier's phase. Often, such circuits provide a precise variable frequency that is derived from an analog signal generated within. Commonly, sine and cosine waveforms are generated that are separately mixed with the detected signal and then filtered to form the in-phase and quadrature phase components of the resulting difference frequency. The in-phase and quadrature phase signals are periodically sampled and converted to digital form for input to the computer that utilizes the information to produce correction signals. The above-described type of circuit commonly includes either a voltage controlled oscillator, voltage controlled crystal oscillator, number controlled oscillator, carrier synthesizer or rate multiplier and incremental phase modulator for generating the sine and cosine waveforms required.
Another common technique involves direct measurement of phase with a phase comparator sampled by the computer. Processing algorithms within the computer replace the sine and cosine generators and other analog circuitry required in the preceding example.
While both of the above mentioned techniques possess certain advantages, the former approach is hampered by a necessity for complex circuitry requiring many discrete components while the latter, although much simpler to implement, is limited by the computational speed of the digital computer which can restrict the sampling rate of the phase comparator.